1,1,1 2-tetrafluoroethane, technically known by the designation 134a, is mainly intended to replace dichlorodifluoromethane (CFC 12) in its applications to refrigeration.
Its preparation by fluorination in the gaseous phase of 2-chloro-1,1,1-trifluoroethane (technically known by the designation 133a) has already been the subject of numerous patents.
U.S. Pat. No. 4,158,675 describes a process for the preparation of 134a by reacting in the vapor phase at a high temperature a haloethane of formula CX.sub.3 CH.sub.2 Cl where X represents Br, Cl or F, with HF in the presence of a chromic oxide based catalyst; the 134a produced, containing 2-chloro-1,1-difluoroethylene as an impurity, is then brought into contact with HF on the same chromic oxide based catalyst, at a temperature between 100.degree. and 275.degree. C. so as to reduce the haloethylene content. The example provided in this patent (trial duration: 3 hours) gives no information on possible recycling of the unconverted 133a and HF reactants.
Patent Application JP 55 27138/80 describes a process for the preparation of 134a by reacting 133a with HF in a molar proportion of 1 in 3 to 20 in the presence of an inorganic chromium (III) compound at a temperature of 300.degree. to 450.degree. C. The examples give no information on possible recycling of the unconverted reactants and the duration of the trials.
Patent FR 2,433,500 describes a process for the preparation of 134a by reacting 133a with HF in the presence of a chromium (III) inorganic compound, characterized in that from 0.002 to 0.05 mol of oxygen per mol of 133a is introduced into the reaction system. The examples give no information on possible recycling of the unconverted reactants. The results obtained during certain trials conducted in the presence of oxygen show stable reaction performances over 85 hours.
Patent Application EP 328,127 recommends, as a catalyst, the use in the presence of oxygen of metals other than chromium, namely Co, Mn, Ni, Pd, Ag and/or Ru on AlF.sub.3, so as to minimize oxidation into chlorine and water of the hydrochloric acid formed leading to a loss in selectivity and a corrosion risk. The examples provided in this patent application give no information on possible recycling of the unconverted reactants. The results obtained during certain trials (carried out with oxygen, on catalysts containing no chromium) show stable reaction performances over 9 hours (Example 1) and 19 hours (Example 2).
The subject of Patent Application EP 331,991 is a process for the preparation of 134a consisting in bringing into contact, in the gaseous phase between 300.degree. and 500.degree. C., 133a and HF on a catalyst containing at least one metal which has an oxidation number greater than zero and is selected from the metals in groups VIII, VII B, III B, I B and/or metals having an atomic number of 58 to 71, then in separating 134a from the leaving flow of gas. The examples illustrating this process give no information on possible recycling of the unconverted reactants. A fall in activity appears after 38 hours (Example 1) or 21 hours (Example 2).
In Patent Application JP 262,946/89 it is indicated that the known methods for maintaining the activity of a fluorination catalyst, such as continuous addition of chlorine or oxygen are not suitable in the case of the fluorination of a halogenated hydrocarbon containing hydrogen (such as 133a) by virtue of the loss of selectivity observed. This publication therefore proposes a process of periodic regeneration of the oxygen catalyst consisting in that, when the activity of the catalyst decreases during the reaction, introduction of the reactants is stopped and a gas containing oxygen is fed to the reaction system in order to reactivate the catalyst, then this feeding with gas containing oxygen is stopped and feeding with reactants is resumed. Examples 2 and 4, and comparative Examples 1 and 2 relate to fluorination of 133a, using catalysts containing chromium, in the presence or absence of oxygen. A fall in activity of these catalysts is noticed. The examples provided in this publication give no information on possible recycling of the unconverted reactants.
In Patent Application JP 172,933/90 whose subject is a process for fluorination of 133a, it is indicated that traditional catalysts, such as chromic oxide on its own, have a low activity at low temperatures and a short lifetime at high temperatures. It therefore proposes carrying out the reaction between 133a and HF in the presence of a fluorination catalyst based on halides or oxides of chromium and on at least one element selected from Al, Mg, Ca, Ba, Sr, Fe, Ni, Co and Mn. It also recommends, together with the specified catalyst, feeding of oxygen or chlorine on the basis of 0.1 to 10 mol % with respect to 133a. All the examples provided use 2 mol % oxygen and are conducted without recycling the unconverted reactants.
Patent Application WO 90/08,755 describes an improved process for the preparation of 134a from trichloroethylene, the improvement consisting in conducting the catalytic fluorination reaction in a single reaction zone fed with trichloroethylene, with HF and recycled 133a, the reaction being able to be carried out in the presence or absence of oxygen. The examples provided are conducted without recycling the unconverted reactants.
Patent Application EP 408,005 describes a process for the preparation of 134a by reacting in the gaseous phase trichloroethylene and hydrofluoric acid in the presence of 133a, the mole ratio of trichloroethylene to 133a ranging from 5/95 to 50/50, and in the presence of a catalyst comprising chromium trioxide supported on AlF.sub.3. The examples provided are conducted without recycling the unconverted reactants, with the exception of Example 6. This example, of 6 hours duration, indicates separation of the components of the reaction mixture into, on the one hand, "light components" of which 134a is one and, on the other hand, "heavy components" and 133a which are recycled to the reactor, but it gives neither the operating conditions of this separation, nor the effect of this recycling on the catalyst, not enabling the example to be reproduced.
Patent Application EP 446,869 describes a process for the preparation of 133a by fluorination in the gaseous phase of trichloroethylene conducted in the presence of inert diluents such as the gases coming from the reaction fluorinating 133a into 134a. Implementation of this integrated process for the preparation of 134a leads, as indicated in FIG. 2 of this patent application, to conducting the fluorination reaction of 133a into 134a in the presence of a recycling of unconverted reactants 133a and HF. However, the conditions for separating the products and reactants and the effects of the recycling are not indicated; the examples provided concern trials carried out in the absence of recycling, feeding with reactants being restored.
Patent Applications EP 449,614 and EP 449,617 describe processes for the preparation of 134a by fluorination of trichloroethylene in two reaction stages (fluorination of trichloroethylene, fluorination of 133a) carried out in series (fluorination of trichloroethylene, then of 133a) or in reverse series (fluorination of 133a, then of trichloroethylene). In both cases, these processes lead to conducting the fluorination reaction of 133a into 134a in the presence of a recycling of unconverted reactants 133a and HF. However, the conditions for separating the products and reactants and the effects of recycling are not indicated; in both patent applications, the examples provided concern trials carried out in the absence of recycling, feeding with reactants being restored and the duration of the trials is not indicated.
In catalytic fluorination of 133a, the degree of conversion of 133a into 134a is limited by thermodynamics. Typically, for an HF/133a mole ratio equal to 2 at the entry of the reactor and a reaction temperature of 400.degree. C., thermodynamic equilibrium corresponds to degrees of conversion of 133a of 20% and of HF of 10%. The flow leaving the reactor therefore contains mostly unconverted reactants (133a and HF) which it is essential to recycle. For this, the main constituents of the flow leaving the reactor can be separated and then purified according to conventional techniques, in particular unconverted 133a and HF, in order to eliminate therefrom, before recycling into the reaction, problematic impurities (such as organic byproducts or water) which are generated in the reaction or brought by the raw materials and which are capable of leading to deactivation of the catalyst or of generating corrosions.